Silicone-base fire resistant hydraulic fluid

ABSTRACT

A fire resistant silicone-base hydraulic fluid having improved antiwear  perties comprising tetrachlorophenylmethyl silicone fluid, dibutylchlorendate and 2,5-bis-alkyl-dithio-1, 3, 4 thiadiazole.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates to improved fire resistant hydraulic fluids andmore particularly to silicone-base hydraulic fluids having improvedantiwear properties.

The currently used petroleum-base, military aircraft, hydraulic fluid,which conforms to Military Specification MIL-H-5606C is an extremelyflammable material. This material, which was developed in the earlyphases of World War II, possesses all of the desired properties of agood hydraulic fluid with the one exception of fire resistance.Hydraulic systems in military aircraft under combat or operationalconditions are subject to projectile damage or component failures whichcould result in the release of this fluid under pressures as high as3,000 psi in the form of a spray or a jet. In the presence of anignition source, a hazardous condition or fire may result, causingaircraft damage or loss and a threat to crew safety. It is clear thenthat military aircraft survivability can be improved by reducing theirvulnerability to hydraulic fluid induced fires resulting from enemyground fire, accidents, and system malfunctions. To this end fireresistant silicone-base hydraulic fluids have been developed. Fireresistant silicone base hydraulic fluids must not only possess thedesirable properties of high flash point and high fire points but mustalso possess other properties which ideally equal or surpass therequirements set forth in MIL-H-5606C. For obvious reasons, one of thechief characteristics such fluids must possess is superlative antiwearproperties.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedfire resistant silicone base hydraulic fluid having significantlyimproved antiwear properties. 3,4 This object and others are achieved byproviding a fire resistant hydraulic fluid which comprises a base oftetrachlorophenylmethyl silicone fluid completely admixed with 2 weightpercent dibutylchlorendate and preferably 0.25 weight percent, 2,5bis-t-dodecyl dithio-1,3,4 thiadiazole.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, there is provided a fireresistant silicone-base hydraulic fluid comprising a complete admixtureof tetrachlorophenylmethyl silicone fluid, 2,5-bis-alkyldithio-1,3,4thiadiazole (preferably 2,5 -bis-t-dodecyl thio-1,3,4 thiadiazole) anddibutylchlorendate.

The additive dibutylchlorendate is a diester of the formula ##STR1## inwhich R is an n-alkyl radical with 4 carbon atoms. Dibutylchlorendatefunctions as a nitrile rubber swelling agent in the formulation of thisinvention and also serves to enhance the antiwear characteristics of theformulation.

The above described diester is readily formed by reacting the carboxylgroups of chlorendic acid with n-butyl alcohol. Chlorendic acid,otherwise identified as 1,4,5,6,7,7 -hexachlorobicyclo(2.2.1)-hept-5-ene-2,3-dicarboxylic acid, is prepared by the Diels-Alderaddition of the corresponding chlorinated cyclopentadiene and maleicanhydride. Chlorendic acid is commercially available from the VelsicolChemical Corporation, Chicago, Illinois.

The thiadiazole used in the practice of the invention is an antiwearadditive which is soluble in the silicone base fluid and which possessesdesirable copper corrosion and thermal stability properties. Thedesignated thiadiazole may be represented by the general formula##STR2## in which R is an alkyl group.

Specific examples of the thiadiazoles falling within the scope of theabove formula, as well is methods of their preparation can be found inU.S. Pat. No. 2,719,125 to Roberts and U.S. Pat. No. 2,983,716 toFields. Thiadiazoles of the type described above are commerciallyavailable from Amoco Chemical Company, Naperville, Illinois.

Specific examples of thiadiazoles which may be employed in theformulation of this invention are as follows:

a. 2,5-bis-t-hexadecyl dithio-1,3,4 thiadiazole

b. 2,5-bis-n-octyl dithio-1,3,4 thiadiazole

c. 2,5-bis-t-dodecyl dithio-1,3,4 thiadiazole

In the formulation of the present invention, t-dodecyl thiadiazole ispreferred because the lubricity characteristics of the formulation aremost enhanced when it is used therein.

The silicon base fluid, tetrachlorophenylmethyl, used in the practice ofthis invention is a co-polymer having 12.5 weight percenttetrachlorophenyl siloxane and 87.5 weight percent dimethyl siloxane. Itis composed of molecules varying in molecular weight from 800 to 6000;approximately 50 percent of which are between 2500 and 3500 in molecularweight.

A tetrachlorophenylmethyl silicone fluid defined by the formula:##STR3##

The fire resistant silicone base hydraulic fluid of the presentinvention is prepared by complete mixing of the three constituents withany conventional means in the presence of heat to approximately150°-200°F. The addition of heat is not essential, however it doesenhance the mixing process.

The following formulation examples are illustrative of the invention andare not to be construed as limiting. In the examples, the lubricity,i.e., antiwear, characteristics are determined by means of the Shell4-Ball Wear test at 167 F, 40 kg, 1200 rpm, for 1 hour; AISI 52100steel. In this test, three balls are locked immovably by a conical ringto thereby form a three point contact for a fourth ball held rigidly atthe end of a drive shaft. The fourth ball rotates, under load, againstthe three stationary balls; all are immersed in the fluid under test.Circular scars are worn on the surface of the three stationary balls andthe degree of wear is expressed as the average diameter of these scars.The smaller the wear scar, the better the lubricating characteristic ofthe fluid under test.

Table I sets forth wear scar results for different proportions of thethree constituents of the formulation of the present invention. It alsotabulates wear scar results where one or both additives are deleted forpurposes of comparison.

                  TABLE I                                                         ______________________________________                                             Silicone             t-dodecyl                                                Base Fluid                                                                              Chlorendate                                                                              Thiadiazole                                         Ex.  (Wt/%)    (Wt/%)     (Wt/%)  Wear Scar (mm.)                             ______________________________________                                         1   99.75     0          0.25    0.82                                         2   99.50     0          0.5     0.83                                         3   99        0          1.0     0.97                                         4   99        1          0       0.93                                         5   98        2          0       0.77                                         6   97        3          0       0.80                                         7   96        4          0       0.79                                         8   95        5          0       0.95                                         9   97.75     2          0.25    0.61-0.64                                   10   98.75     1.0        0.25    0.72                                        11   96.75     3.0        0.25    0.69                                        12   97.9      2.0        0.10    0.67                                        13   97.6      2.0        0.40    0.74                                        14   94.75     5.0        0.25    0.79                                        15   94.50     5.0        0.50    0.80                                        16   94.00     5.0        1.00    0.81                                        17   100.00    0.0        0.0     1.22-1.35                                   ______________________________________                                    

Formulation example 9 indicates the preferred proportions of theformulation constituents since its unexpectedly low wear scar valuesdemonstrate that it has superior lubricating qualities. These qualitiesare not lost with rising temperature as demonstrated by the resultstabulated in Table II wherein Formulation examples 1, 5 and 9 weresubjected to the same 4-Ball Wear Test but at an increased temperature(350°F).

                  TABLE II                                                        ______________________________________                                                  Wear Scar (mm.)                                                     Example     167°F   350°F                                       ______________________________________                                        1           0.82           0.95                                               5           0.77           1.38                                               9           0.61-0.64      0.68                                               ______________________________________                                    

Obviously many variations of the present invention are possible in thelight of the above teachings. It is therefore to be understood thatwithin the scope of the appended claims the invention may be practicedotherwise than as specifically described.

We claim:
 1. A silicone base fluid having improved lubricitycharacteristics comprising:a major amount of a tetrachlorophenylmethylsilicone fluid defined by the formula ##EQU1## wherein thetetrachlorophenyl siloxane and the dimethyl siloxane moieties arepresent in amounts of 12.5 and 87.5 weight percents, respectively; anamount in the range of from 0.1 to 0.4 percent by weight of2,5-bis-alkyl-dithio-1,3,4 thiadiazole; and an amount in the range offrom 1.0 to 3.0 percent by weight of dibutylchlorendate.
 2. A siliconebase fluid according to claim 1 wherein said alkyl thiadiazole isselected from the group consisting of t-hexadecyl, n-octyl and t-dodecylthiadiazole.
 3. A silicone base fluid having improved lubricitycharacteristics comprising:substantially 97.75 weight percent of atetrachlorophenylmethyl silicone fluid defined by the formula ##STR4##wherein the tetrachlorophenyl siloxane and the dimethyl siloxanemoieties are present in amounts of 12.5 and 87.5 weight percents,respectively; substantially 2.0 weight percent dibutylchlorendate; andsubstantially 0.25 weight percent 2,5 bis-t-dodecyl dithio-1,3,4thiadiazole.